This invention relates to a transporting system and more particularly, to a transporting system for transporting an exposure master plate, such as a reticle or a photomask.
Using an exposure apparatus (e.g., a stepper), a circuit pattern formed on a reticle is exposed to light so as to transfer the image of the circuit pattern onto a wafer coated with a photoresist layer, thereby forming the circuit pattern on the wafer. In general, multiple layers of circuit patterns are superimposed on the wafer during exposure, and a plurality of different types of reticles having different patterns are successively used during the exposure process.
In order to interchange the multiple reticles promptly, several types of transporters that are capable of transporting master plates (e.g., reticles) in an efficient fashion have been proposed. For example, Japanese Patent Application Laid-Open No. 7-321179 and corresponding U.S. Patent Application Ser. No. 08/445,170 disclose such a transporter that retrieves a master plate from a container, carries the master plate from the container to the exposure stage, and returns the master plate into the container. The container is installed in an exposure apparatus to accommodate one or more master plates.
With this type of transporter, it must be confirmed in advance (i) that the master plates are actually stored in the container, and (ii) which master plate is stored at which position. To this end, the operator has to check that the master plates are in the container, the positions of the master plates, the type and the capacity of the container, and so on. In the automatic transporting mode, the operator generally specifies the positions of the master plates and gives directions for transporting the master plates.
In this context, the operator may make a mistake in his directions. For instance, if the operator mistakenly gives a direction to retrieve a master plate from an empty position, a transport error occurs because the transporter moves to the empty place to receive a nonexisting master plate. In such a case, the operation of the exposure apparatus and the production line stops. Conversely, if the operator issues directions to return the master plate to a position at which another master plate is stored, the two master plates strike each other, and the expensive master plates can be damaged.
Transport errors may also occur when a container is replaced with another container that stores different master plates in different positions. If the operator forgets to update the directions for the new container, a master plate that is being returned to an empty position in the former container may collide with another master plate that is stored in the same position in the other container. This then is another type of scenario in which the master plates can be damaged.
To eliminate transport errors caused by the operator, optical sensors for detecting whether a master plate is stored in each position of the container have been proposed. Reticles, however, which serve as the master plates, are made of a transparent material, such as quartz, and are made in the shape of a flat plate (referred to as a quartz glass plate). Because most of the light passes through this quartz reticle, it is difficult for a transmission-type optical sensor to determine its presence, i.e., the presence of a master plate.
Furthermore, several types of containers, including a single-plate container and a multiple-plate container, are used in an exposure apparatus. The automatic transporting mode cannot be successfully performed unless the type of container is identified. If a direction is made under the assumption of a multiple-plate container when a single-plate container is used then a transport error or a collision between two master plates may occur.